Radars perform double duty as high-speed data links

Synthetic aperture radars have used radio frequency technology to give aircraft, ships and ground troops highly detailed tracking data. Now, they might provide a way to share that data in real time. Contractors Raytheon and L-3 Communications have combined efforts in a joint development program that might turn synthetic aperture radar systems into nodes on a high-speed, mobile ad hoc network.

Using the radar’s antennas simultaneously for radar sensing and as a high-speed data link, fighter aircraft would be able to transmit full sensor data — previously only available within the aircraft — to other aircraft and ground stations more than 100 miles away. If successful, the capability that Raytheon and L-3 are developing might transform fighter aircraft and other vehicles equipped with Active Electronically Scanned Array (AESA) radars into powerful intelligence, surveillance and reconnaissance (ISR) platforms, sending synthetic aperture radar images at speeds as fast as 4 gigabits/sec.

“The data that [fighter aircraft have] gathered, which is extremely valuable, has been limited to use in that cockpit because there was no way to offload that amount of data,” said Lucas Bragg, Raytheon’s senior manager of advanced programs. “By now enabling their radar to act as a communications device, you're now able to offload this highly valuable data that's been gathered on the aircraft.”

“The big thing with this technology is that fighters have been limited in getting large amount of data off the vehicle, because you'd have to add an aperture, an antenna,” said James Perry, L-3's director of international business development.

“With the sleek skin of the aircraft, there's no way to add an antenna that will give you the throughput to do wideband communications.”

By adding a modem, a small box or some cards underneath the skin and then using the switching of the antenna to toggle between communications mode and regular radar mode, fighter pilots can connect to other communications assets in theater, he said. “If they're using AESA radars to be able to send wideband data off of that aircraft, [then] basically, the [synthetic aperture radar] images that the pilot can see there can now be seen on the ground.”

As part of a joint independent research and development program, Raytheon and L-3 are developing a system that would allow pilots to simultaneously use radar as a sensor and data link. Depending on the requirements, part of the radar’s array could be dedicated to a continuous data link directed to a command and control ground station or aircraft while the remainder functions as a sensor. Or the communications could be sent in pulse mode, sending data between radar scans.

Entering the network

The joint program pulls together technologies the two companies have developed through independent research and development and through research that the Defense Advanced Research Projects Agency contracted. The mobile ad hoc network, or Manet, “has been developed over several years primarily in support of DARPA programs, specifically a program called Future Combat Systems Communications (FCS-C), which went on for about six years,” Bragg said.

“What that encompasses is a mobile network capability that allows the network to autonomously develop and control itself without the intervention of users,” Bragg said. “For example, when you have multiple vehicles out there, including aircraft — it's aircraft to ground — as vehicles approach, they automatically enter the network, they establish themselves, who their neighbors are, and the communications are configured autonomously.”

The Manet also includes built-in quality-of-service elements that help it maintain links in less-than-desirable conditions, he said. The Manet technology, called Raymanet, was first demonstrated in 2006 as part of DARPA’s network-centric demonstration at Fort Benning, Ga. “We've been running this Manet, with different radios and different modems, at much lower data rates” than the radar-based Manet, Bragg said. “But the network is a field-certified product. We’re fielding it with other customers at this time.”

The second part of the equation is L-3’s modem for the AESA radar. “This is the device that allows the movement of data across this network,” Bragg said. L-3’s current AESA modem, in its fourth generation, provides an extremely high data rate, Bragg said. “They've demonstrated 4.5 gigabits/sec, which for an airborne or ground-to-ground link is enormously fast — a record-setting speed for this kind of device.”

L-3 has been testing high-speed radar-based data communications with Raytheon and others for some time. AESA-based data communications have been evaluated in connection with the F-22 Raptor program. L-3, Northrop Grumman and Lockheed Martin began working on a system for a data link for the F-22 fighter in 2005. The companies demonstrated a 274 megabits/sec Common Data Link connection using a CDL emulator modem and the AESA radar aperture for the F-22 fighter in January 2006. The capability, called Radar-CDL, was also tested at 1 gigabit/sec in throughput.

Also in 2006, L-3 began working with Raytheon to demonstrate AESA data links using the Raytheon Multiplatform Testbed (RMT) aircraft, a Boeing 757 that is configured to allow different nose cones to be attached to it.

“This Raytheon RMT aircraft, the unique feature with it is that they can put different nose cones on that that are the same radars as the F-15 or the F-18,” Perry said. “So we could see what it would be like from the point of view of a fighter aircraft.”

The RMT was tested over Catalina Island in California and beamed data to a ground station about 125 nautical miles away, Perry said. In that test and others, the system was able to use a pulsing radar signal to achieve 274 megabits/sec throughput, he said.

Real-time surveillance

A similar demonstration, using L-3’s third-generation modem technology, was shown at the Milcom military communications conference in San Diego in November 2008. The fourth generation of L-3’s modem, now in testing, has achieved 4.5 gigabit/sec transmission rates. And earlier this year, the pulse-based transmission capability was demonstrated as part of another DARPA project, called the Affordable Adaptive Conformal Electronic-scanning-array Radar (AACER), Perry said.

AACER is a real-time tactical surveillance system that uses a low-cost, low-weight AESA radar that can be installed in a helicopter or rotary-wing unmanned aerial vehicle, such as Boeing’s A-160 Hummingbird. A Blackhawk helicopter was used during the test as a stand-in for a UAV, using the radar to capture surveillance data and transmit it to a ground station, Perry said.

The next major step in testing involves taking the AESA-based Manet to a lower speed for a ground-based ISR application. Raytheon is looking at combining the AESA Manet with its Long Range Advance Scout Surveillance System (LRAS3) to provide a way for vehicles to send ISR imagery to a tactical operations center. “The ISR data that is being collected but not distributed in the aircraft world, well, it’s true in the ground world as well,” Bragg said.

This summer, Raytheon and L-3 will go into the lab with a proof-of-concept system that ties LRAS3 into an AESA radar developed by Raytheon for the Future Combat Systems program. Known as the Multi-Function Radio Frequency System (MFRFS), the technology was developed for the FCS Active Protection System program. “The AESA for MFRFS comes from the same design as that for our F-18 radar program,” Bragg said. "It's a very similar, much lower cost radar system for ground use.”

“We're going to be demonstrating [the AESA Manet] with a ground-to-ground application, so it would be from vehicle to vehicle to vehicle, and all of the vehicles with that capability could share that data, as well as move it back to the tactical operating centers," Bragg said.

Potentially, with the wider application of FCS technology, the MFRFS system could be deployed on vehicles throughout the Army. Bragg said Raytheon and L-3 plan to move into a field demonstration of the technology this fall.